1. Field of the Invention
The present invention relates to amplification in a signal transmission line.
2. Related Art
A line driver is an amplifier that amplifies a transmission (“TX”) signal and drives the TX signal along a signal line. The line is characterized by its impedance, Zline. Typically, line drivers have to drive large amplitude signals into low ohmic loads and hence are high power amplifiers. Additionally, for voltage mode line drivers there is a back matching impedance Zt, also known as the termination impedance. Termination impedance Zt must be matched to the line impedance Zline to have no reflections. For a 1:1 transformer, for example, the actual voltage at the output of the line driver is approximately twice as big as the transmission voltage driven on the line.
In full duplex transmission systems, the same pair of wires carries the TX and received (“RX”) signals simultaneously. The signal that includes both the TX and RX signals is called the line signal. Because of the number of signals, duplex transmission systems need a 3-port (TX, RX, and Line) circuit that can separate the RX signal from the TX signal. This 3-port circuit is called a hybrid.
Many systems use a single set of hybrid elements which are chosen to best match the line impedance under most conditions. But, since the line impedance may change depending on various properties of the line, such as load or length of the line, etc., current hybrids do not sufficiently separate the RX signal from the TX signal. Instead, a residual TX signal called the TX echo often remains in the extracted RX signal. This can degrade the quality and signal-to-noise ratio of the RX signal.
One solution is to use multiple sets of hybrid elements that are switched depending on the characteristics of Zline. This requires multiple inputs for the front end and multiple sets of hybrid components that result in extra cost. Multiple components also require switches that choose the best component set for a particular signal line. These switches typically degrade linearity since they are highly nonlinear.
Therefore, there is a need for an efficient system that achieves improved TX echo cancellation in the hybrid.